RFID tag sensitivity

ABSTRACT

Systems, methods, or computer program products may collect information about the performance of individual RFID tags. In particular, signal strength and/or signal sensitivity of individual RFID tags may be measured and the resulting performance information may be stored in a data repository. In the data repository, the performance information for an individual tag may be associated with an identifier that is uniquely associated with the tag. In one exemplary embodiment, the performance information may be used by an on-line system configured to automatically determine the performance margin with which RFID tags are being read in an RFID system, or to automatically tune the RFID system to achieve a desired performance margin.

PRIORITY CLAIM

This application is a continuation-in-part of, and claims priority to,U.S. patent application Ser. No. 10/936,972, filed on Sep. 9, 2004 andentitled “RFID System Performance Monitoring,” and to U.S. patentapplication Ser. No. 10/936,971, filed on Sep. 9, 2004 and entitled“RFID Sensor Array,” both of which are hereby incorporated by referencein their entirety.

TECHNICAL FIELD

This invention relates to monitoring the performance of RFID systemsusing sensitivity information for individual RFID tags.

BACKGROUND

Radio frequency identification (RFID) systems are capable of wirelesslydetecting individual RFID tags (herein referred to as “tags”) as theypass in the vicinity of an RFID reader. Unlike bar codes, which are readindividually and must be in the optical line-of-sight of the scanner,RFID tags can rapidly read a large number of tags that are within aregion around a reader, such as a number of tagged products that aretogether on a shelf or a pallet. Moreover, tags may be read through somematerials, such as paper or plastic packaging material used to shipcases or pallets of products. As such, individual products may be taggedand read after being placed in a container for storage or shipment.

RFID tags may be wafer-thin, microchip-based tags about the size of alarge postage stamp. The tags may include antenna features capable ofreceiving radio frequency energy to communicate with a reader.Typically, a reader may detect a tag by transmitting radio frequency(RF) energy at a particular frequency and with a particular modulation.A tag within the appropriate range of the reader may be energized by theRF energy received via the tag's antenna. The energized tag may respondby generating or reflecting an RF signal that may be detected by thereader's antenna and receiver. The signal sent by the tag may be encodedwith information, such as a serial number, electronic product code(EPC), stock keeping unit (SKU), or other identifying information. Afterthe reader receives and decodes the identifying information, thepresence of the tag may be recorded or stored for various uses. Whenindividual tags are associated with products in a distribution chain,for example, uses may include, but are not limited to, inventorymanagement, product tracking, billing, loss prevention, or ordering.

Product tracking in a distribution chain may use RFID systems to trackinventory, for example, at various control points in a manufacturingplant, distribution centers, warehouses, and retail outlets. In apractical large scale RFID system, not all tags may be read. In thedistribution chain context, RFID system performance may generally referto the reliability with which tags are read as products move through thedistribution chain.

At each of these points, the margin by which tags are satisfactorilyread is subject to various parameters, such as temperature and humidityconditions, electromagnetic interference (EMI), reader/tag sensitivity,material properties, and numerous other factors that can impact howreliably tags are detected by the RFID system. Accordingly, reliablytracking tags in such environments requires the RFID system to operateunder various atmospheric, electromagnetic, and other conditions thatcan impact RFID system performance.

SUMMARY

Systems, methods, or computer program products may collect informationabout the performance of individual RFID tags. In particular, signalstrength and/or signal sensitivity of individual RFID tags may bemeasured and the resulting performance information may be stored in adata repository. In the data repository, the performance information foran individual tag may be associated with an identifier that is uniquelyassociated with the tag. In one exemplary embodiment, the performanceinformation may be used by an on-line system configured to automaticallydetermine the performance margin with which RFID tags are being read inan RFID system, or to automatically tune the RFID system to achieve adesired performance margin.

In an illustrative example, individual tags may be tested to determine asensitivity level that is associated with the tag. This tag sensitivityinformation may be combined with other collected information tostatistically determine the performance margin at individual nodes inthe RFID system. Sensitivity information may be determined as a functionof a tag's placement on a particular product, or within a containercontaining one or more products. Such sensitivity measurementinformation may be used to improve the accuracy with whicherror-contributing parameters may be identified in the overall RFIDsystem.

In one aspect, a method of collecting performance information forindividual RFID tags includes measuring the performance of a tag, andstoring one or more values that represent the measured performance in aninformation repository. The method further includes associating thestored values with an identifier that is uniquely associated with thetag.

In an embodiment of the method, measuring the performance of the tag mayinclude stimulating the tag with radio frequency (RF) signals at leastonce at each of a number of field strengths. The measuring furtherincludes attempting to read the tag's response to stimulation at each ofthe number of field strengths, and determining a first field strength atwhich the tag is successfully read at an error rate that is less than afirst predetermined error rate. In addition, the method may furtherinclude determining a second field strength at which the tag is notsuccessfully read at an error rate that is less than the firstpredetermined error rate. Still further in the method, the first andsecond determined field strengths may differ by less than a firstpredetermined amount, and a first of the values that represent ameasured performance is based on the first and second determined fieldstrengths. Similarly, additional field strengths may be determined foradditional values that represent measured performance.

Various embodiments of the method may also include other features,including: reading the tag at least once at each of a plurality ofdifferent spatial positions relative to a reader, such as by placing thetag at different locations or in different spatial orientations; varyingthe power level transmitted by a reader; or encoding the tag with a codethat represents the identifier that is uniquely associated with the tag.An RFID reader may also perform the method.

According to some embodiments, reading a tag successfully may includereading the tag without errors at least a predetermined number of timeswhen a number of attempts are made to read the tag. In still otherembodiments, measuring the performance of the tag may include measuringthe strength of a RF signal transmitted by the tag in response to thetag receiving RF signals associated with an attempt to read the tag, andthen storing one or more values that represent the measured signalstrength in an information repository. The stored values of signalstrength may be associated with the identifier that is uniquelyassociated with the tag.

In another aspect, a computer program product is tangibly embodied in aninformation carrier and contains instructions that, when executed, causea processor to perform operations to determine the sensitivity of aradio frequency identification tag. One operation may be to measure thesensitivity of a tag by stimulating the tag with RF signals at leastonce at each of a plurality of field strengths and attempting to readthe tag at each of the plurality of field strengths. Another operationmay be to store one or more values that represent the measuredsensitivity in an information repository. Yet another operation may beto associate the stored values with an identifier that is uniquelyassociated with the tag.

In yet another aspect, a method of commissioning an RFID tag for use inan RFID system that tracks the movement of article through a supplychain includes measuring the performance of an individual RFID tag andencoding the RFID tag with a unique identifier. The method also includesassociating the unique identifier with a first information representingthe measured performance, and storing the first information and theunique identifier in an information repository. The method furtherincludes coupling the tag to an article to be tracked as the articlemoves through the supply chain.

In some embodiments of the method, measuring the performance of anindividual RFID tag includes measuring the performance of the tag afterthe tag has been coupled to the article to be tracked. The method mayalso include estimating a power level sufficient for one of the RFIDreaders to read the tag based on the stored first information andadjusting a reader power level according to the estimated power level.The estimated power level may be sufficient to read the least sensitivetag in a group of tags. Estimating a power level sufficient for one ofthe RFID readers to read the tag based on the stored first informationmay further include determining a first power level that represents theminimum reader power level required to successfully read the tag at anerror rate that is less than the first predetermined error rate, anddetermining the estimated power level by adding a predetermined marginto the determined first power level.

Some embodiments may provide one or more advantages. For example, thesystem may provide for the collection of information related to theperformance of the RFID system. Tags may be individually measured atcommissioning to verify tag performance early in the life cycle, andcorrective action may be taken before additional investment is made inreliance on tag performance. Performance information for individualtags, as well as tag lot codes, versions, and other tag-relatedinformation may be compiled to quickly and accurately detect or predictthe impact of tag parameters on RFID system performance. Informationcollected by the system may be compiled to facilitate either manualand/or automated analysis to identify sources of disturbances to RFIDsystem performance, as well as to suggest or implement correctiveactions to improve performance. The compiled information and analysismay be used by system architects, engineers, and operators to gaininsight into performance margins in the RFID system, what factors limitthe performance margins, and how the RFID system, the environment, orthe operation of the RFID system may be improved.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a functional block diagram that includes an RFID performancemonitoring system.

FIG. 2 is a process flow diagram that may use the RFID performancemonitoring system of FIG. 1.

FIG. 3 is a flowchart of a method used with the RFID performancemonitoring system.

FIG. 4 is a functional block diagram illustrating one aspect of dataflows and data processing in an RFID performance monitoring system.

FIG. 5 is a flowchart of a method of applying RFID system performancedata to improve RFID system performance.

FIG. 6 is a flowchart of a method of analyzing performance data todetermine parameters that impact system performance.

FIG. 7 is a top view of a distribution center in which an array ofreaders monitors sources of electromagnetic energy.

FIG. 8 is a flowchart of a method of using inactive readers to monitorother readers in the RFID system.

FIG. 9 is a flowchart of a method of measuring the sensitivity of a tag.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

In one exemplary embodiment, individual tag sensitivity information maybe used to enhance an RFID performance monitoring system. An RFIDperformance monitoring system may collect a variety ofperformance-related information that relates to tracking RFID taggeditems as they move though a distribution chain. The performancemonitoring system may analyze the collected information for the purposeof identifying parameters that contribute to reduced reliability of RFIDsystem performance. In various embodiments, the performance monitoringsystem may perform statistical operations on the collected informationto determine their impact on system performance. Providing sensitivityinformation for individual RFID tags may reduce the uncertainty in thestatistical analysis, thereby improving the quality and speed of theoutput of the RFID performance monitoring system.

In various embodiments, information that is collected for analysis mayinclude performance metrics, such as, for example, tag sensitivitylevels, successful reads and read attempts for individual tags.Information may also be collected about product configuration, tagplacement and orientation information, time, temperature, humidity,vibration, power line quality, and the like. Information may also becollected about the lot code, version, supplier, and history of tags,readers, and other hardware and software. Furthermore, information maybe collected about the product to which each tag is attached, such asinformation about tag trajectory and velocity, cases, pallets, materialsinvolved, and SKU of products. In addition, information about individualtags may include tag design, vendor, calibrated sensitivity,orientation, version, antenna, lot code, and the like. Other parametersthat may relate to system performance may also be monitored, such as thestate of a dock door (open or closed), or the energization of a motor,for example.

An RFID performance monitoring system may include systems, methods, orcomputer program products for collecting information related to theperformance of an RFID system. Generally, RFID system performancerelates to a measure of how reliably readers in the system identifytags. To assess parameters that may affect RFID system performance,information from a variety of sources may be collected for analysis. Thecollected information may be analyzed using statistical methods or othertools to identify patterns that indicate sources of error relative toRFID system performance. Accordingly, information may be collected aboutoperational results, system elements, and the environment within whichan RFID system may operate. The performance related information may becorrelated with other information, such as product information, RFID tagor reader information. Each tag may be uniquely associated with anidentifying label, which may allow collected information to beassociated with individual tags. The monitoring system may also processthe information for the purpose of taking some corrective action. Assuch, the system may serve as an analytical tool for identifyingopportunities to improve RFID system performance.

Many factors can degrade RFID system performance. For example, tagfailure may occur. The RFID hardware and software, the atmosphericconditions, tag and reader designs and variability, the trajectory ofrelative motion between the reader and the tag, and the physicalcharacteristics of the area around the reader are also examples offactors that may affect the reliability with which tags are read in anRFID system.

In some embodiments, a tag may be attached to an individual article, orit may be attached to a container of articles to be tracked by the RFIDsystem. The tag may be attached to an article that has a fixed location,such as a shelf in a distribution center (i.e., or other warehouse), orit may be attached to a mobile article. One example of a mobile articleis a forklift. Another example of a mobile article is a consumer (orother) product that may be moved through a supply chain via conveyor,forklift, truck, hand carry, or similar mode of conveyance. The supplychain may include any segment of product flows from an originalequipment manufacturer (OEM), through various distribution centers, toretail outlets, and to the end consumer. Along the supply chain, one ormore RFID systems may be used to track the movement of tagged articlesthrough the supply chain for various business purposes, such asminimizing loss, improving inventory management, or reducinginventory-related costs, for example.

These business purposes may be best achieved by using a high performanceRFID system that reliably reads all tags as they pass by readers. Inpractical systems, however, there are many potential reasons that areader may fail to perform, i.e. fail to read 100% of tags that pass by.One way to enhance the business purposes of the RFID system involvesidentifying factors that hinder performance. If parameters thatnegatively impact performance can be identified, then mitigationstrategies may be developed and corrective actions taken to improve RFIDsystem performance.

An RFID performance monitoring system may be configured to identifyparameters that correlate with REID system performance. An RFIDperformance monitoring system may be configured to collect, analyze, andshare information related to the performance of the RFID system. RFIDsystems at various points along the supply chain may share informationwith other RFID systems. Performance information may be analyzed toidentify conditions that may improve or degrade RFID system performance,and how reliably tags are read by the RFID system.

One measure of the degree of reliability with which tags are read isreferred to herein as performance margin. The greater the performancemargin, the lower is the likelihood that tags will not be read properly.As an illustration, if a reader is able to properly read all tags in apallet at a particular power level, then the performance margincorresponds to how much the reader power level may be reduced before thereader performance would fall below a predetermined and acceptablethreshold.

After sources of error or reduced performance margin have beenidentified, system planners, managers, engineers, and technicians maydesign and implement mitigation strategies and take corrective actionsto improve performance. In some embodiments, the mitigation strategiesmay include closed-loop feedback control systems configured toautomatically take corrective actions without human involvement. Inother embodiments, human involvement may be required to perform aspectsof the analysis or to take corrective actions.

For ease of understanding, the several aspects of an RFID performancemonitoring system will be introduced after a discussion of conventionalaspects of an RFID system that is capable of tracking the movement ofarticles. Next, details of various aspects of operating an RFIDperformance monitoring system are presented. Then, additional featuresthat may provide enhanced functionality for the performance monitoringsystem will be explained.

RFID System for Tracking the Movement of Articles

Beginning with FIG. 1, an exemplary RFID system 10 is configured totrack the movement of RFID tags (“tags”) that are attached to articlesto be tracked. Similar to a conventional RFID system, the RFID system 10is coupled to one or more RFID readers 12. Each reader 12 may include anantenna and a controller that are configured to detect tags within aread field using radio frequency (RF) signals. The RFID system 10 alsoincludes an RFID operations server 14 that may perform functions relatedto tracking the movement of articles. In this example, the operationsserver 14 communicates with the readers 12 through a reader interface16, and it is coupled to a number of information repositories that storeinformation related to tracking the movement of articles. In thisexample, the information repositories include a product database 20, atag database 22, and a tag list database 24.

To provide for tracking the movement of articles, the operations server14 in this example is also coupled to a middleware server 30 that mayexchange information with external systems, such as the Internet 32and/or an intranet 34, for example. Over the Internet 32, the RFIDsystem may have access to a source of unique object names, such as anobject name service (ONS) 36. The ONS 36 may provide a unique code suchthat each tag used in an RFID system may be encoded with informationthat uniquely identifies that RFID tag. This tag identifyinginformation, which may take the form of a special code (e.g., 64 or 96bits), may be stored in the tag. One example of such a code is, forexample, an electronic product code (EPC). EPCs may be applied to tagsto provide each tag with a universally unique serial number. Tags mayalso be programmed with other information such as, for example, the partnumber, lot code, manufacturer, or stock-keeping unit (SKU) for thearticle to which the tag is attached.

Through the intranet 34, the RFID system 10 may exchange information,such as business, accounting, and inventory location information, with awarehouse management system (WMS) 38 a. At the WMS 38 a level, thecomputing system may monitor and control one or more independent RFIDsystems, including RFID system 10. As such, the voluminous datacollected by each reader in each RFID system may be summarized forreporting purposes, for example. Multiple WMS's, such as 38 a and 38 b,may be coupled to a firm's information technology (IT) system so thatthe RFID tracking data may be integrated with other corporate andhigh-level management functions. Some RFID information may be madeavailable to other firms in the supply chain. For example, the RFID datamay be sent, for example, over a virtual private network (VPN), forstorage and retrieval by business partners (e.g., suppliers,distributors, transporters, retailers), such as a firm 40 b, that haveaccess to aspects of a shared RFID database 50. Although an exemplaryarchitecture has been described, other network architectures andimplementations may also be used.

The foregoing description introduced various aspects of an exemplarysystem configured to conventionally track movement of articles using anexemplary RFID system with an array of RFID readers. With thatbackground, the performance of such an RFID system may be monitored by aperformance monitoring system that will next be described.

RFID Performance Monitoring System

The illustrated RFID system 10 of FIG. 1 further includes an RFIDperformance monitoring system 100 (hereinafter, “system 100”) that maycollect information from the RFID system and other sources. Thecollected information may be analyzed, either by the system 100, or byhuman operators. One purpose of analyzing the collected information isto identify opportunities for improving the performance of the RFIDsystem 10.

The system 100 includes a processor 110 that is coupled through a bus toa memory 120 for information storage and retrieval. The memory 120 mayinclude program storage memory 122, a compiled database memory 124, astatistical analysis memory 126, and an auxiliary memory 128. Over thebus, the processor 110 may retrieve program instructions stored in theprogram storage memory 122. The processor 110 may execute the retrievedprogram instructions and thereby perform functions related to thecollection, processing, compilation, analysis, or storage of performancerelated information. In some embodiments, the program instructions mayfurther cause the processor to take corrective actions to improve RFIDsystem performance, and/or to report processed analytical results toexternal systems or to human operators.

The processor 110 may be coupled to various devices that receive, send,store, display, or process data and control signals. The connectionsbetween the processor 110 and these devices may be, for example, direct(IC to IC), intra-chip (i.e. integrated in a single IC or ASIC), over aparallel bus including address, data, and control lines, over a serialbus (e.g. USB, Ethernet, or controller area network), or a combinationof such methods. The signals to and from the processor 110 may includeinterrupts, control, and hand-shaking signals for operating aprocessor-based performance monitoring system. Various architectures maybe used to implement the functions of receiving, storing, processing,and transmitting information. The processor 110 may be implemented in asingle microprocessor or microcontroller, or it may incorporate multipleprocessors programmed to provide a coordinated operation of thefunctions described herein. In one embodiment, some operations may behandled by a main microprocessor that delegates the processing ofcertain analytical functions to a math co-processor, for example.

In this example, the processor 110 is also coupled to a sensor interface140 and a feedback control interface 150. The sensor interface 140 maybe coupled to a number of parametric sensors 142 from which the system100 may collect information about parameters that may affect theperformance of the RFID system 10. For example, the parametric sensors142 may be used to monitor temperature, relative humidity, vibration,power line quality, operating states of various equipment, such aswhether a dock door is open or closed, or whether a certain device(e.g., motor) is active or inactive. Other parameters, such asenvironmental or other phenomena that may potentially introducedisturbances into the reading of tags may also be monitored. One exampleof such a sensor may include an RF receiver for detecting and monitoringambient RF energy that may disrupt the reading of RFID tags.

The feedback control interface 150 may be configured to send a number ofcontrol signals 152 to effect corrective action(s) to improve RFIDsystem performance, and/or to report processed analytical results toexternal systems or to human operators. For example, one control signal152 may be coupled to one of the readers 12 to allow the system 100 toadjust the power level of the RF signal transmitted by that reader 12.On one hand, the reader power level may be reduced, for example, toreduce unintentional interference with other readers and to avoidreading tags that are not intentionally within the read zone of thereader. On the other hand, reader power level may be increased toincrease the likelihood of reading all tags in the read zone of thereader. Whether power level should be increased or decreased to improveRFID system performance may be determined from analysis of the datacollected by the system 100.

The processor 110 is also coupled to a performance database 160 and aparameter database 162. In this example, the databases 160-162 mayprovide a repository for data collected by the system 100. For example,the processor 110 may receive performance-related data from the RFIDoperations server 14, process the data using methods that will bedescribed below, and store the results from the processing into theperformance database 160 or the compiled database 124. When handlingeither product or performance-related data (stored in databases 20-22)that is associated with a particular tag, the processor may store theprocessed information in a way that uniquely associates the informationwith a stored tag identifier, such as an EPC.

The processor 110 may store information received from the sensors 142 inthe parameter database 162. In some embodiments, information collectedfrom the parametric sensors 142 may be associated with a particularreader in the reader array 12, for example, while other information maybe associated with positional or other reference information, such as aparticular dock door in a distribution center, or a particular piece ofequipment. Such associated information may be analyzed with othercollected information, for example, to locate the source of RFID systemperformance problems.

Collected information may be compiled into the database 124. Theinformation compiled may be processed or filtered according toprogrammed instructions executed by the processor 110. The compiled datamay be stored directly into memory 124 at the time it is collected, orit may be stored or derived from information initially stored in anotherdatabase, such as databases 20-24, 160-162, or other (external) databaseaccessible by the processor 110 via the Internet 32 or the intranet 34.Other information may be received from information stored on aninformation carrier, such as a floppy disk, CD, flash card, or tape, orother data storage medium or device. In some embodiments, theinformation may be compiled “online,” being effectively added to thecompiled database 124 as it is being received by the RFID system 10 inreal time. In some examples, the processor 110 may perform “off-line”processing of the compiled data in the database 124.

In certain embodiments, the processor 110 may execute programinstructions stored in the program storage 122 to process theinformation as it is being compiled into the compiled database 124. Someprocessing may associate the information being received with otherinformation, such as EPC numbers, time information, locationinformation, or other information so that analysis may reveal meaningfulrelationships between parameters. Some processing may filter theincoming data, for example, to reduce redundancy or to eliminatenon-useful (e.g., null value) information. Other processing may includedeveloping indices or other metadata on certain parameters or fields inthe database by which database searches may be improved. Otherpre-processing may be performed to format or build the compiled databasefor analysis. In some embodiments, time information (e.g., a time stamp)may be associated with some information as it is being added to thedatabase. While compiling information into the database 124, theprocessor 110 may request supplementary information from other sources,such as the WMS 38 a-38 b, or the IT 40.

After processing the data in the compiled database 124, the processor110 may store some of the results in a memory location such as thestatistical analysis report memory 126. The information stored thereinmay be formatted and structured for convenient review and access byvarious utilities, such as graphical display programs for generatinggraphical displays on a display device viewable by an operator. Thegraphical output may be updated continuously in real-time, at periodicintervals, or as requested by an operator. Longer-term or historicalresults information may be reviewed by the operator using, for example,a computer terminal or other input/output device for accessing thereporting information. Such displays may incorporate graphical outputcapable of illustrating trends and status information in variousgraphical, tabular, or other reporting formats. Alarm conditions may bedisplayed to indicate that parameters or performance-related informationor processed results fall outside predetermined acceptable limits.

In addition to visual display information, advanced reportingcapabilities may be included in the system 100. For example, the system100 may be configured to send various levels of compiled or processedinformation and results to the middleware server 30, the WMS 38 a-38 b,and the IT firm 40 a. The information may be sent on a routine basis, orin response to requests. The information may also be sent when theprocessor 110 detects certain predetermined conditions. For example, iftags associated with a particular manufacturer lot code are determinedto have less than satisfactory performance, then a message may begenerated and sent to the WMS 38 a-38 b or Firm IT 40 a-40 b to notifyappropriate personnel to, for example, reject further use of tags fromthe unsatisfactory lot code. In a related example, the system 10 maysend a control signal 152 to increase the power level and/or the numberof reads attempted by readers 12 that may be attempting to read tags ofthe unsatisfactory lot code. As illustrated by these examples, a numberof different corrective actions may be taken when a performance-relatedproblem is identified.

As an example of locating the source of a performance problem, sensors142 may be configured to measure temperatures at various locations inthe environment of the RFID system 10. In one embodiment, at least sometemperature sensors 142 are in close proximity to some of the readers12. By collecting the temperature around readers 12 at various locationsin a distribution center, for example, the impact of temperature on theperformance of individual readers may be determined. After identifyingspecific readers that have temperature-related performance problems, thesource of a performance problem may be located. This locationinformation may provide additional bases upon which to select correctiveactions. In this example, potential corrective actions may include:re-routing the flow of articles to readers that are less sensitive totemperature-related performance degradation; replacing the affectedreader or tags with ones that are less sensitive to temperature;relocating the reader to reduce temperature fluctuations (e.g. away fromdock doors); providing local temperature regulation for the reader(e.g., cooling fan, heat lamp, or other HVAC equipment); adjustingreader power level to compensate for temperature variations; increasingthe number of read attempts; varying the physical product/tagconfiguration on pallets; or, changing the type of tags being used.These examples represent some potential mitigation strategies that maybe used to reduce the effect of temperature variations on a particularreader whose performance has been identified as sensitive to temperaturevariation. As such, this example illustrates how corrective actions maybe based on an analysis of the impact of various parameters collected bythe system 100.

In various embodiments, the RFID system 10 and RFID performancemonitoring system 100 may be configured differently than theillustrative embodiment of FIG. 1. For example, various elements may beimplemented on a single computing platform, such as a single server,desktop or laptop computer. Alternatively, certain of the variouselements in the RFID system 10 and the RFID performance monitoringsystem 100 may be implemented in distributed computing platforms and mayuse other hardware and software, including servers, PCs, laptops,mainframes, programmable logic controllers (PLCs), hand held computingdevices, interfaces, and the like. As such, the elements may exchangeinformation using wired and/or wireless communication protocols, such asUSB, Bluetooth, RS-232, Ethernet or other communication methodsinvolving, for example infrared, RF, or fiber optics. Similarly, thedatabases 20-24 and 160-162 may be implemented in a single data storagedevice, along with the memory elements 120-126, or in any combination ofseparate storage devices. Storage devices may include any suitablememory devices such as, for example, disk drives, flash devices, EEPROM,RAM, or ROM.

RFID Performance Monitoring in an Exemplary Supply Chain

The RFID performance monitoring system 100 may be used to monitor theperformance of RFID systems in various applications. For purposes ofillustration, FIG. 2 represents a series of stations in an exemplaryembodiment of a portion of a supply chain in which an RFID system maytrack the movement of articles, and a performance monitoring system 100may collect performance-related information. This example may represent,for example, an RFID system that is used to track the movement ofarticles in a distribution center in which various goods may bereceived, stored, and placed on pallets for shipment to retailcustomers, for example.

In this example, the sequence of stations begins with the receipt ofproducts, items, or articles to be tracked 205 into the RFID system. Inone embodiment, the sensitivity of the tag may be determined at astation 210. Sensitivity may be determined at the station 210 (or otherstation) using various methods that will be described in detail withreference to FIG. 9. In some embodiments, a station 220 may be aprogramming station that programs each tag with an identifying serialnumber, such as an EPC code. The commissioning station 220 has a reader215 that may be configured to “program” the tag with the assigned EPC,for example. At the commissioning station 220, the tag may further beuniquely associated with information about the product to which it isattached (or otherwise coupled to). As such, an operator may enter theproduct information and EPC information for storage in a database, suchas the databases 20, 22, by using a computer terminal that is coupled tothe RFID system 10.

Some or all of these functions at stations 210, 220 may be performed“upstream” of the distribution center. For example, the manufacturer mayapply tags to the product, program the tags with an EPC code, anddetermine the sensitivity of the tag. The manufacturer could then makethis information available to downstream RFID systems by storing theinformation in the shared RFID database 50, or by otherwise distributingthe information with the distribution center, such as by VPN or email,for example.

Accordingly, information to be programmed into the tag may be sent fromthe RFID system 10 to the station 220, and optionally may include othertag or product information. In one embodiment, the RFID system 10 or theRFID performance monitoring system may send configuration information,for example, that instructs the operator how to apply the tag to aparticular product. Such information may reflect a corrective actionresulting from an analysis of performance-related information thatdetermined that tags on a particular product could be read better bychanging the location or orientation at which a tag is applied to theproduct, for example.

Furthermore, information about the tags and their associated productsmay be sent from various stations to the RFID system 10 and/or thesystem 100. Once received, this information may be stored in thedatabases 20, 22, for example. Other information may also be receivedfrom various stations, such as a tag list. A tag list may comprise alist of all the serial numbers in a single group of tags that are readby a reader. The RFID system 10 may compare the received tag list withan expected list of tags to determine if products may be missing. TheRFID performance monitoring system 100 may further cross-reference thetag list with other information to determine if tags are not beingdetected due to performance-related issues.

Other stations in the distribution center may send and receiveinformation with the system 100. In this example, the tags are next readat a station 225, where products are aggregated into pallets (or similarshipping container) and wrapped in plastic for shipping. The pallets maybe mixed (including various different products) or homogenous (all oneproduct type). In mixed pallets, the composition, orientation,materials, and configuration of the products within the pallets may beuncontrolled. As such, the location of RFID tags on individual productsmay vary, and the readability of tags may be subject to the materials onthe pallets and the location of the tags within the pallet of goods.Accordingly, the performance monitoring system 100, in some embodiments,may exchange configuration-related information with the station 225.

As the product is being wrapped, for example, on a rotating platform(i.e. spin table), one or more readers may be attempting to read thetags on the pallet. As such, the tag list of read tags may be providedto the RFID performance monitoring system 100. The efficacy of thereadings of pallets on the spin table may be a function of severalparameters, such as spin speed, number of rotations, palletconfiguration, and temporal multiplexing of multiple readers arrayedaround the spin table at the station 225. As suggested above withreference to FIG. 1, the system 100 may include control output signals152 that are coupled to the spin table controller and the readers tooperate the station 225 to improve RFID read performance withoutunnecessarily slowing the operational flow of the pallets through to thenext station.

In this example, the next station may be for stocking the pallet on theshelf 230 for subsequent shipment. When ready to ship, a forklift maypick the pallet from the shelf 240. In some embodiments, the pallet isread by a reader associated with the shelf, or a reader 215 on-board theforklift as it is picking or moving the pallet. An on-boardcommunication and computer system may collect and transmit the tag listfor the pallet to the RFID system 10 for verification purposes, and/orto the system 100 for data collection purposes. As such, the forkliftcomputer, operator, the on-board reader, and the RFID system 10 mayexchange command, data, and control information, for example, usingwireless communications.

In some embodiments, the product flow includes reading the products witha reader 215 in a verification tunnel 250 just before being placed on atruck to be shipped, for example. Tracking the movement of articles asthey are placed on trucks may have significance, for example, tobusiness processes, such as insurance and invoicing. However, accurateverification of items being shipped may be complicated by numerousfactors, including: variable reflections off of moving objects, such asforklifts and other metal items; changes in temperature and relativehumidity as the dock door 260 opens or closes; mechanical vibration;radio frequency noise in the environment of the readers; interferencefrom readers of adjacent dock doors; and, other sources of error. Todetect these parameters at any of the reader stations, various sensors142 may be configured to provide parametric information to the RFIDperformance monitoring system 100.

At any station that provides communication to the RFID performancemonitoring system 100, the information received from the station mayinclude timing or time stamp information. Temporal information provides,in some examples, for determining historical trends, or for correlatingperformance changes with particular parameters, for example. To theextent any performance parameter variations depend on time, such timingevent information may be used to identify with greater accuracy sourcesof performance-related disturbances.

In an alternative product flow, referred to as tag-at-ship, the productmay proceed directly from aggregation at station 225 to the dock door260 without being placed on the shelf 230.

Accordingly, FIG. 2 represents one of many possible paths for goods toflow through the exemplary distribution center, and it depicts only oneconfiguration of RFID stations in the distribution center or otherwarehouse. In other examples, a performance monitoring system maycollect performance-related information with other configurations of thesequence, number, or operation of the RFID stations, as well as inapplications or environments other than a supply chain or a distributioncenter. For example, the sensitivity of an individual tag may bemeasured after the tag has been commissioned and applied to a product atthe station 220. As such, the sensitivity measurement would reflect thesensitivity of the tag as it will be while attached to the product. Thissensitivity measurement after application would also take into accountthe orientation and placement of the tag, as well as any affect theproduct itself may have on the tag's sensitivity due to factors such asfluid content, shape, or metal content. In another example, the tagsensitivity may be measured both before and after commissioning the tagwith a product at the station 220.

In addition to sending tag list information to the system 100, eachstation in FIG. 2 that reads the tags may also send one or moreperformance-related metrics, such as the following: time to read 90% ofthe tags in the tag list; time to read 100% of the tags in the tag list;and, the number of successful reads and number of read attempts for eachtag. Other percentages of time may be used, such as time to read 10%,25%, 50%, 66%, 75%, and 80%, or other value. A time limit may be placedon some or all of the time metrics, such as time to read 100% because,for example, a particular tag may fail or may be unable to be read dueto a failure, or a reader may be unable to read effectively. In someembodiments, a reader or set of readers may be configured to read agroup (such as a pallet) of tags for a predetermined number of attempts,and record the number of times each tag is successfully read. Timestampinformation may be included with the performance-related informationprovided to the system 100.

An exemplary method by which an individual RFID reader may develop suchperformance-related information while reading tags is described in theflowchart of FIG. 3.

In the FIG. 3 embodiment, a reader receives a list of tags for a groupof tags that are to be read at 310. The group of tags may be part of apallet of products, for example. At 315, the reader may make apredetermined number of attempts to read all tags in the group. Thepredetermined number of attempts may be 10 or 20, for example. At 320,the reader reports the number of successful reads, also referred to as“hits,” for each tag to the RFID performance monitoring system 100. Withreference to FIG. 1, this reported information may be stored, at leasttemporarily, in the performance database 160.

Next, at 325, the tag list of successfully read tags may be compared tothe list of tags expected to be in the pallet (or other container orgrouping of tags). If less than a predetermined percentage of expectedtags have been read, then continued attempts may be made to read theadditional tags at 330. If at least the predetermined percentage hasbeen read, then the “time required to read the predetermined percentage”may be reported at 335 to the system 100, and may then be stored in theperformance database 160. If less than all of the expected tags havebeen read at 340, then a timing system is checked to see if apredetermined time limit for making read attempts has been reached at345. If the time limit has not been reached, then additional attempts toread all the tags may be made at 350. If no time remains at 345, thenthe read attempts end at 370. If, however, all of the tags have beenread at 340, then, at 360, the reader reports the “time to read 100%” tothe RFID performance monitoring system 100, and the read process ends at370. This “time to read 100%” may be stored with other performancemetrics in the performance database 160.

In another embodiment, the reader may not receive a list of tags thatare expected to be read as described above at 310. Instead, the readermay attempt to read the group of tags a predetermined number of times,and then send a tag list containing the EPC (or other identifyinginformation) for each detected tag to the RFID system 10 and/or to theperformance monitoring system 100. In addition, the reader may sendinformation to the system 100 from which the system 100 may determinethe performance for the read attempt(s). Such information may include,for example, the number of read attempts, time stamp information aboutwhen the attempts were made, and which attempts were successful for eachtag.

In the above-described example, certain algorithms were described asbeing performed within the reader. In alternative embodiments, somecomputations or decisions may be performed, for example, by or incooperation with the RFID performance monitoring system 100. Forexample, the time needed to read X % at 335, or whether the time limithas been reached at 345, may be determined by the system 100. Theattempts to read all tags at 330, 350 may be modified to involve thesystem 100 sending read commands to the reader. The predetermined numberof attempts at 315 may be determined by the system 100 based on, forexample, an expected error rate, and appropriate commands may begenerated to the reader. These and other variations are within the scopeof the method.

In some embodiments, for example, the RFID system 10 may send serialread requests to a “non-intelligent” reader, which may send a tag listof tag hits for a single read cycle in response to each command. Inother embodiments, the RFID performance monitoring system 100 may send amore complex command that tells an “intelligent” reader how manyattempts to make, to which the intelligent reader responds with a taglist and a count of the number of hits of each tag after performing therequested number of reads. Some intelligent readers may be configured toreceive, decode, and perform basic and auxiliary functions (describedherein) in response to commands from the RFID system 10 or the RFIDperformance monitoring system 100. Intelligent readers may, for example,be capable of performing certain functions that may otherwise beperformed by the middleware server 30.

In various implementations, readers may be activated to perform readoperations by other sensors such as optical sensors on a conveyor,motion detectors, proximity detectors, or by operator actions. Trackingthe entry and exit of tags or groups of tags (e.g., pallets) into andout of the read field of a reader may involve filters and algorithmsthat estimate or determine the length of time during which the tags maybe read or should be ignored. For instance, a forklift may have apermanently mounted tag (or other beacon or identifier) that uniquelyidentifies the forklift. When that forklift's proximity to a reader at adock door is recognized, middleware running on the middleware server 30can be configured to determine which tags are expected to be present onthe pallet on that forklift (for example, by calling on informationstored in the WMS 38 a). The RFID system 10 can use this information tofilter valid tag reads from stray reads.

Processing Information in the Performance Monitoring System

In accordance with the foregoing description, the performance monitoringsystem 100 may operate to receive input information and transmit outputinformation as shown in FIG. 4. In this exemplary embodiment, the system100 may collect information, analyze the information, and providesoutputs that are based on the analysis.

The system 100 may receive a variety of information as inputs from arange of sources. One source of information is auxiliary information410, which may provide information from the Internet 32, or fromservices such as WMS 38 a and the middleware server 30. Auxiliaryinformation 410 may include information that may be stored on datastorage devices accessible over a network, for example.

Collected auxiliary information may include information about hardwareor software associated with the IT, WMS, middleware, RFID operationsserver, the intranet, or other networked elements. In addition,auxiliary information may include vendor, version, lot code, or otherinformation about the hardware or software used in tags, readers,communication interfaces, and the computing platforms used in the RFIDsystem 10. The hardware or software used in these elements can have animpact on the performance of the RFID system. As such, the system 100may collect auxiliary information about hardware or software vendors,lot codes, versions, maintenance data, upgrade history, and installationinformation. Such information may be correlated with changes in theperformance of the RFID system 10. For example, hardware changes mayaffect the amount or quality of data that is collected.

In one embodiment, the auxiliary information may include informationabout the location of forklifts over time. The location information maybe wirelessly transmitted from a position tracking system on-board theforklift. Such location information may be correlated with RFID systemperformance to determine if the position of a forklift can have adetrimental impact on RFID system performance. If it does, proceduresmay be developed to mitigate the negative impact.

The system 100 may also receive information from auxiliary sensors 415and environment parameter sensors 420. Examples of auxiliary sensorsinclude proximity sensors that detect parameters that may affect readerperformance. Such parameters may include, for example, the approach offorklifts or other objects in the vicinity of a reader, the position ofparticular dock doors (i.e., whether open or closed), or the operatingstate of large machinery that may produce heat, vibration, orelectromagnetic interference (EMI).

Examples of environmental parameter sensors 420 may include temperature,humidity, vibration, power line quality, or radio frequency devices.Such environmental parametric sensors may be arranged to collectinformation about the environment in which the RFID system is operating,and which are believed to have an impact on RFID system performance.

The system 100 may also receive information from each reader 12. Theinformation provided by readers may include tag lists, performancemetrics, sensitivity information about each reader itself, informationabout ambient RF energy (see discussion of FIG. 7), information aboutthe power levels of other readers (see discussion of FIG. 8), andinformation about the sensitivity of individual tags (see discussion ofFIG. 9).

One method of monitoring and/or improving reader power and tagsensitivity measurements includes providing a “golden” target, i.e. atag of calibrated sensitivity that is placed in the read zone of areader. In some implementations, measurements of golden tags may be usedto compensate the sensitivity measurement of an uncalibrated readerbased on the reader's sensitivity measurement of the “calibrated” goldentag(s). As an example, a golden tag may be measured as having a 10 dBsensitivity if measured in a lab with calibrated equipment, but may bemeasured as having an 8 dB sensitivity if measured by an uncalibratedreader in a factory environment.

Various approaches may be used to identify compensation values that maybe applied to compensate measurements from uncalibrated readers. In oneexample, the reader may attempt to read the calibrated golden tag atvarious power levels to determine what power level is required for thereader to read each of the golden tags at an acceptable “hit rate.”Other exemplary methods of measuring reader sensitivity and power levelsare described, for example, with reference to FIG. 8.

Based on diagnostic measurements of one or more golden tags, the systemmay determine calibration information for each uncalibrated reader. Insome embodiments, the calibration information may be post-processed intoa set of compensation factors and/or stored, for example, in a memorylocal to each reader, or in a database remote from the reader.

Once calibration information has been determined, the information may beapplied to realize improved sensitivity measurements. In one embodiment,the calibration information may be used to adjust a tag parameter, suchas a field strength measurement, or reader parameters, such as gain,attenuation, frequency response, or power level, for example.Compensation information may also be used to adjust the sensitivity testprocedure (see, e.g., FIG. 9) parameters such as, for example, thedistance(s) and/or locations of the tags relative to the reader whenmaking sensitivity measurements. The calibration information may also beapplied to computations performed in software or firmware, includingadjustments to digital filters (e.g., FIR, IIR) or other digitallyprocessed computations. Accordingly, compensating the measuredsensitivity values to correct for reader calibration errors may improvethe accuracy and uniformity of tag sensitivity measurements within thesystem. More accurate sensitivity measurements may yield improvedaccuracy in determining performance metrics associated with individualRFID tags, as well as improved overall performance of the RFIDperformance monitoring system 100.

Performance metrics for individual tags may include, for example, thenumber of successful reads of a tag from a certain number of attempts;and, the measured tag sensitivity (see, e.g., the station 210 in FIG. 2,and also FIG. 9). Performance metrics for individual readers may includethe time to read a predetermined percentage of tags in a tag list,measured reader sensitivity, and measured reader power levels.

The RFID performance monitoring system 100 may initially collect all theperformance-related data and information received from sources, such asthose described above, in a data collector 430. In some embodiments, thedata collector may be implemented as a single database, or as a numberof repositories for collecting raw information for later processing, asdescribed above with reference to FIG. 1.

The system 100 may, in some embodiments, be configured to process thecompiled data 124 using various analytical tools. Such analytical toolsmay include statistical tools capable of producing outputs thatrepresent the degree of correlation between parameters. For example,statistical tools may include software capable of computing correlationand/or covariance factors for collected parameter information andperformance-related information. Other statistical tools may be used inthe analysis, including, but not limited to: minima, maxima, mean, andregression analysis techniques.

In one embodiment, a statistical analysis engine 440 determines whichparameters need to be analyzed, retrieves the parameter data from thecompiled data 124, performs an analysis, and interprets the resultsaccording to programmed instructions. In another embodiment, thestatistical analysis engine 440 may be implemented by programinstructions executed on the processor 110 (FIG. 1).

The mathematical computations may be executed on the processor 110, oron a math co-processor, such as a digital signal processor (DSP) orother computing element capable of analyzing the compiled data 124 as acorrelation and covariance calculator 445, in this example. After thecompiled data has been analyzed, parameters that exhibit highcorrelation factors with one or more of the performance metrics may bemarked for reporting, corrective action, or other further scrutiny.

Further scrutiny is one example of an outcome that may result inresponse to the output of the system 100. As a further example, if theanalysis yields as a result that performance is uncorrelated to any ofthe measured parameters, then a system operator may determine, forexample, that sensors should monitor different parameters. This isanother example of a response to the output of a report generator 450.

The output from the report generator 450 may be sent to a display 452,such as a printer, or on a display terminal that may include a graphicaluser interface (GUI). The GUI may incorporate charts, graphs, or otherdata or values monitored by the system 100. Another output of the reportgenerator may be data that is sent to the middleware server 30, and/orto the WMS 38 a. The reports may be generated in response to a requestfrom these elements, on a regularly scheduled basis, or in response toan alarm condition, such as when a monitored value crosses apredetermined threshold limit. A scheduler/prioritizer module 455 mayassign the schedule and priority of outputs. The module 455 may alsoassign priority and arbitrate the commands to the controller 460.

The controller 460, which may include the feedback control interface 150(FIG. 1), may include interfaces, drives, and control elements that maybe coupled to control the operation of one or more devices in the RFIDsystem, or in the environment that may affect RFID system performance.The outputs of the controller 460 may control the operation of auxiliaryequipment 470, and may control the spin table 470. In addition, thecontroller 460 may be adapted to control some of the readers 12 bycontrolling their power levels and, where readers may interfere with oneanother, by controlling the timing (i.e. multiplexing) of operation ofreaders 12. The controls signals may be transmitted to programmeddevices, such as PLCs, computers, or other industrial control devicesand equipment.

The auxiliary equipment 470 controlled by the controller 460 mayinclude, for example, dock doors, heating, cooling or humidity controls,or the operating state of various equipment that may impact RFID systemperformance. In one embodiment, the auxiliary equipment may includeindicators or displays that notify distribution center operators aboutcurrent keep-out zones around individual readers. For example, if aparticular reader is exhibiting a low performance margin, then adisplay, such as on a positioning system on-board a forklift, mayindicate to the forklift operator to maintain certain keep-out zones,which should not be entered whenever that reader is operating.

As another example, a dispenser may be operated to dispense articlesonto a conveyor at a controlled distance of separation so that readersreading items on the conveyor are less likely to unintentionally readtags that are near a “read zone.” Likewise, the controller 460 maycontrol the conveyor speed to achieve maximum throughput at anacceptable level of RFID tracking performance.

The controller 460 may also be configured to command the operation ofthe spin table 475 according to the results of the performance analysis.The control commands may determine the angle of rotation of the spintable, the number of rotations, and the speed of rotation.

In another example, an antenna may be mounted to a vertically orientedpositioning system near the spin table. A motor drive may be configuredto control the vertical position of the antenna in response to controlcommands from the RFID system 10. The control commands for antennavertical position may be related to the spin commands. As the spin tablerotates a pallet with a number of tags, for instance, the verticalposition of the antenna may be changed to maximize the readingeffectiveness for tags in the pallet.

The system 100 may monitor which trajectories of vertical position androtation exhibit the best performance for different pallets. Forexample, some dry goods pallets may be read faster than pallets thatcontain significant fluid and/or metal content. Based upon accumulatedperformance-related information, the optimal control commands may beapplied to maximize RFID performance at the spin table. Furthermore, foreach pallet type, the system 100 may be operated to tune spin tableoperating parameters, such as spin rates, vertical antenna motiontrajectory, antenna power settings, and antenna multiplexing sequences.If multiple readers are provided to read tags on the spin table 475,then the controller 460 may configure, for example, the number of readattempts, power levels, and the sequence at which the readers aremultiplexed. These configuration parameters may be customized and storedin libraries according to pallet type. These libraries may be recalledas needed to achieve maximum performance based on previous analysis andresults for the particular pallet configuration, tag and readerconfigurations, and products in the pallet.

Information Analysis and Corrective Action

The RFID performance monitoring system 100 may be operated using theexemplary method shown in the flowchart of FIG. 5. Additional detailsfor an exemplary method of operating the statistical analysis engine 440are then described in the flowchart of FIG. 6.

The flowchart in FIG. 5 starts with collecting data at 515. In oneembodiment, the data collector 430 may collect the data received by thesystem 100. The data is compiled into data sets that may be associatedwith time information at 520. In this example, the system createsperformance indices at 525. Each performance index may includeperformance metrics for a predetermined number of previous reads, andmay include information for each reader, such as time to read 90% andtime to read 100%. During analysis, each performance index may becorrelated with various parameters to identify potential causes ofsub-optimal performance.

As will be described in greater detail with reference to FIG. 6, thesystem 100 may perform an analysis at 530 to determine which parametersmay have a significant impact on the RFID system performance. In analternative embodiment, a human operator may perform some or all of theanalysis. Based on the analysis, the system 100 may select parametersfor corrective action and place the selected parameters on a mitigationlist at 535. The system 100 may then prioritize and schedule correctiveaction at 540 by using, in one embodiment, the scheduler/prioritizermodule 455. The system 100 may then apply the corrective action at 545.The corrective action may be implemented as changes to the RFID system10 at 550, as changes to the environment in which the RFID system 10operates at 555, or as closed-loop feedback at 560. In alternativeembodiments, part or all of the steps 530-555 may be performed by humanoperators.

Although the system 100 may identify some RFID performance problemsafter a relatively small number of samples have been collected, some ofthe corresponding corrective actions 550, 555 may have a relatively longtime frame. For example, changes to the system 550 may involve repairingor replacing readers or tags, or redesigning the system to add, remove,or modify the type or location of readers, or changing the routes ofproduct flows through the distribution center, for example. Some changesto the system 550 may involve hardware and/or software revision and/orinstallation. Some changes to the system may involve the antenna designof either the tag or the reader, operating frequency, software of thereaders and/or the middleware server 30, and the like. Other changes mayinclude changing the orientation or placement procedures by which tagsare attached to items. Product packaging, placement, content, andpalletization are all subject to improvement in response to the analysisof the RFID system performance.

Similarly, changes to the environment 555 may involve reducingtemperature or humidity variations around readers, for example. Suchchanges may involve changes to the physical plant so that the localenvironment around the readers may be controlled. In one example,cooling fans and/or heat lamps may be provided to control the variationof temperature and humidity around readers. In another example, dockdoors may be configured to better shield the RFID system from outsideambient EMI sources, such as police cars, communications systems,airplanes, and the like.

The closed-loop feedback at 560 may be implemented using the controller460 to provide corrective actions that may be automated using feedbackcontrol methods or that may be implemented in a relatively short timeframe. Some examples of elements that may be controlled by feedbackmethods include: dock doors; fans; spin table (speed, number ofrotations); reader multiplexing; and, reader power levels.

The functions of the steps 525-535 of the method of FIG. 5 will next bedescribed with additional detail in the exemplary method shown in FIG.6.

The method involves testing sets of parameters stored in the compileddatabase to identify parameters that may have an impact on performance.Starting with the compiled datasets that were created in step 520, thesystem 100 may select a set of one or more parameter(s) for evaluationat 610. Next, the system 100 calculates at 615 a covariance of the setof selected parameters and each performance metric in the performanceindices that the system 100 created at step 525. The system 100 thendetermines the variability of the selected parameter at 620. Forexample, the variability may be measured as the peak-to-peak variationof the selected parameter over a period of time. At 625, the system 100computes the impact of each parameter on each performance metric bymultiplying the covariance computed at 615 with the variability of theselected parameter determined at 620.

In this example, the system 100 compares the computed impact for eachperformance metric to a predetermined threshold at 630. If the impact isgreater than the predetermined threshold, then the system 100 adds theselected parameter to a proposed mitigation list at 640. After 640, orif the impact is less than or equal to the predetermined threshold, thesystem 100 checks whether more parameters remain to be evaluated at 645.

If more parameters remain to be evaluated, then the system 100 selectsthe next set of parameters for evaluation at 650, and the evaluation ofthat set of parameters begins back at 615. Otherwise, the process endsat 655.

The proposed mitigation list may be sorted, for example, by the impactcalculated at 625. In one embodiment, the scheduler/prioritizer module455 may rank the impacts and assign the highest priority to thoseparameters having the greatest impacts on performance. The reportgenerator 450 may display or otherwise report the highest priorityparameters. The controller 460 may take corrective actions to addressthose parameters having the highest levels of impact on performance.

However, some parameters may be highly observable but not inexpensivelycontrolled. For example, humidity may be inexpensive to measure butexpensive to control directly. Humidity sensitivity is primarily causedby the variations in cardboard packaging moisture content changing itsradio frequency properties. As an alternative to directly controllinghumidity to which packaging is exposed, the sensitivity of readers (orother equipment) to humidity may be reduced by changes to the readerdesign.

As a further example, it may be determined that the humidity isdegrading the signal by changing the characteristics of the products towhich the tags are attached. Accordingly, operators may evaluate thefeasibility of package changes, or different applications of tags to theproducts, to improve RFID performance under high humidity conditions.

Potential corrective actions in response to high impact parameters maybe further illustrated by another example. If low temperature at areader is correlated to a 25% impact (decrease) in performance marginfor that reader, the controller 460 may increase the reader power levelto compensate for the decreased performance margin. Furthermore, if suchan increase in the power level of that reader may result in interferencewith a nearby reader, the controller 460 may also constrain the readersto multiplex, i.e. stay silent while the other reader is operating.Although this may reduce throughput of goods while the temperature islow, the RFID performance level may be maintained, along with thebenefits of RFID tracking.

Other mitigation rules may be programmed into the scheduler/prioritizermodule 455. In some applications, throughput may be more valued morethan RFID system performance. In some instances, the cost functionsassociated with an available corrective action may only be justifiedunder certain conditions. For example, the energy costs associated withclimate control may only be justified for certain products for whichRFID tracking accuracy is deemed highly important. Accordingly, thescheduler/prioritizer module 455 may evaluate weighted rules todetermine which corrective actions to forego, and in assigning priorityto corrective actions.

RFID Readers as Sensors

In addition to the role of RFID readers 12 in the RFID system 10 atstations (see FIG. 2) at which tags may be tracked, RFID readers mayalso serve as an array of RF sensors. When not actively reading tags,readers may be configured to “listen” with their antennae as receiversof ambient RF energy. Information about the received RF ambient energymay be collected by the RFID performance monitoring system 100.Operation of readers as an array of sensors is illustrated in FIG. 7.

In FIG. 7, a distribution center 705 includes an RFID system operated bya central computer station 710 with an RFID performance monitoringsystem. The RFID system includes readers 715, 720, 725, and 730. Thereader 730 is configured to read tags on pallets placed on a spin table735.

Readers 715-730 may be configured to provide information about ambientRF energy to the performance monitoring system during periods when eachreader is not actively reading tags.

For one example, a source of RF energy inside the distribution center705 moves sequentially from point A to points B, C, and D. The source ofRF energy may be a radio transmitter on-board a forklift or carried by asecurity guard.

At point A, the signal strength measured by reader 715 will berelatively strong, and will weaken as the source moves to point B, C,and D. The signal strength measured by readers 720 and 730 will increasebetween points A-B, peak between points B-C, and fall off between C-D.The magnitudes of the signal strength received by each reader may berecorded at several instants in time. From the known positions of eachreader, and the relative strengths of signal received by each reader,the approximate location of the RF source may be determined bytriangulation. From this information, an approximate path over time forthe RF source may be ascertained. This information may be compiled withother information as a parameter that may be correlated with performancemetrics to identify whether the RF source may impact the performance ofthe RFID system.

For a second example, a source of RF energy outside the distributioncenter 705 moves from point E to F. The source of RF energy in this casemay be the radio transmitter on a vehicle, such as a forklift, a truck,or a police car. In addition to such mobile sources, typical stationarysources of RF energy may include, for example, RF welders, electricmotors and drives, lighting systems, and the like.

As in the previous example, the performance monitoring system 100 maymonitor the reader 725 to detect the presence of RF energy in thefrequency bands of interest to the RFID system. By detecting thepresence of RF energy that, for example, peaks in signal strength at thesame time some performance metrics dip, an operator may be able todetermine that RF energy event(s) may be causing the disturbance, andthen proceed to investigate the source of the RF energy. If the sourceof the EMI can be identified, then a solution may be developed tomitigate the effects of the disturbance. For example, if an RF welderthat operates five times per hour on first shift were to cause tags tobe missed, then a potential corrective action may be to pause readingfor thirty seconds whenever that disturbance signal is detected.Alternatively, power levels may be increased and multiplexing adjustedto avoid reader collisions.

In one exemplary method, a database may be created to includecharacterization information about the relative signal strengthsreceived by one or more readers when monitoring an RF source at variouslocations around an RFID system. For example, in a distribution center,relative reader sensitivity levels may be characterized by taking an RFsource to various known locations and recording the relative signalstrengths received by readers in the system. The characterizationinformation may later be used to help locate the source of an RFemission by comparing the relative received signal strengths of the RFemission to the characterization map.

In another embodiment, the characterization information may be used, incombination with other information, to determine whether sensitivity ofindividual readers have drifted since the characterization map wasdeveloped. Characterization maps developed at different times may alsobe compared to identify changes in reader sensitivity levels.

Reader characterization may also include using an RF source of knownpower level and at known locations to determine absolute sensitivityinformation about readers. By characterizing sensitivity of one or morereaders at various known locations around the distribution center, forexample, performance-related information about the sensitivity of eachreader may be used to enhance the analytical results obtained from theperformance monitoring system 100.

Using Readers to Monitor Readers

Not only can RFID readers be operated as a passive array of sensors forcollecting information about ambient RF energy, each RFID reader may beactively operated in a way that allows the performance monitoring system100 to collect information about other readers in the RFID system 10.Such operation may enable the system 100 to identify reader-relatedperformance problems, or to detect performance trends. One particularproblem includes nearby readers interfering with each other, which iscalled “reader collision.” An exemplary method for operating readers tomonitor each other is shown in the flowchart of FIG. 8.

An array of readers may be used to identify relative changes in powerand sensitivity levels of readers in the RFID system as follows. Themethod starts with the system 100 selecting a reader during a period inwhich other readers are inactive at 810. The selected reader is operatedto transmit an identifying RF signal (i.e., chirp) at 815, which maycomprise a serial number that uniquely identifies the reader. Then, at820, the performance monitoring system 100 may receive information aboutthe signal strength of the “chirp” received by each reader that islistening. Based on expected and historical values, the system 100determines if the transmitting reader is transmitting power in a validrange at 825. Next, the sensitivities of listening readers are checkedat 830 to determine if any of the readers received an unexpectedly highor low signal strength value.

Continuing with the procedure, the performance monitoring system 100 maythen check for interferences between readers at 835, whereby a neighborreader may be negatively affected by the interfering signal. Moreover,the performance monitoring system may determine whether one reader mayinadvertently read tags in the read zone of a neighbor reader.

Based on the measured interference levels, the performance monitoringsystem may adjust the multiplexing (i.e., timing) and/or the powerlevels of individual readers at 840. The RFID performance monitoringsystem, or an operator, may constrain the interfering readers to operatein a multiplexed mode so that they do not attempt to read tags at thesame time. In addition, power levels may be decreased to reduceinterference between readers, or increased to improve performance.

If readers remain to be checked at 845, then another inactive reader isselected back at 810. If no readers remain to be checked, then theprocedure ends 850. The procedure may be repeated as readers experiencesufficient intervals of inactivity to check at least one of the readers.

This scheme may be used to generate a “reader collision map.” A readercollision map may include for each reader a list of the readeridentification numbers and signal strengths it received from otherreaders. As such, this may comprise a two-dimensional reader collisionmap that can be updated in real time to adjust reader multiplexing. Thereader collision map may also be used as an analytical tool to identifyundesirable reflections that may be occurring from walls or otherobjects. For example, the two-dimensional reader collision map may beoverlayed on a physical site map to design and place screens to reducethe RF coupling between interfering readers. For another example, readertiming/multiplexing may be adjusted to control reader collisions.

Tag Sensitivity Information

Knowing the sensitivity and power levels of each reader providesadditional information that may be correlated with RFID performancemetrics to identify parameters that impact performance. As mentionedabove with reference to the station 210 of FIG. 2, the sensitivity ofeach individual tag may be determined according to a method described inthe flowchart of FIG. 9. To the extent that RFID tags of a particularlot code, version, and vendor may have sensitivities that range in valueaccording to a distribution curve, information about the sensitivitiesof individual tags may enable the RFID system to assessperformance-related information more accurately. For example, thebelow-average sensitivity of an individual tag may help to explainperformance-related data that shows below-average performance associatedwith that tag, and thereby the source of related errors may be moreaccurately identified.

In the flowchart of the exemplary method of FIG. 9, the process ofmeasuring tag sensitivity begins by selecting an individual tag fortesting at 910. The tag is configured to receive a predetermined signalstrength from the reader at 920. The configuration refers generally tothe RF signal power at the tag, although tag orientation with respect tothe radiation pattern of the reader, antenna design, and other objectsthat affect the radiation pattern at the tag may affect the actualreceived signal strength. In general, however, the predetermined signalstrength may correspond, for example, to the RF energy received by aparticular tag in a particular location and orientation relative to aparticular reader operating at a particular power level. In oneembodiment, the predetermined signal strength is achieved by placing thetag at a known position and orientation relative to a reader beingoperated at a known power level.

The reader attempts to read the tag at 930. If the tag is readsuccessfully at 940, then the signal strength received by the tag isreduced at 950, and the reader again attempts to read the tag at the newpower level at 930. For example, the distance between the tag and readermay be increased, the orientation of the tag may be changed, or thereader power level may be reduced in increments. The increments may belinear, logarithmic, or other relationship to achieve the desired levelof accuracy.

In another embodiment, the signal strength may be determined using othermethods. For example, the signal strength at 950 may be either increasedor decreased to implement a search strategy to determine the tagsensitivity within a range. The search strategy may involve, forexample, a search pattern, such as those well known to those skilled inthe art of computer programming. For example, using a variation of thegolden section search method, the reader power may be adjusted halfwaybetween end-point values between which the desired value is known tolie. For example, if the read is successful at the half-power level,then the power is decreased to halfway between the half-power level andthe lower end-point (which may be initially set to zero). If the read isnot successful, then the power is increased to halfway between thehalf-power level and the upper end-point (which may be initially set tomaximum power of the reader). This may be repeated until the sensitivityis bracketed between end-point values that are within the desired degreeof accuracy.

In addition to improving the identification of sources ofperformance-related errors, the tag sensitivity information may providefurther benefits. For example, tag sensitivity may be used to assess theperformance margin of individual readers with a greater degree ofprecision. Based on a statistical analysis of performance-relatedinformation that is adjusted for the sensitivity of individual tags, theperformance margin of individual readers may be monitored, weak elementsin the system identified, and corrective actions taken before failuresoccur.

The concept of performance margin may be expressed in various ways. Inone aspect, performance margin in an RFID system may be considered to bea measure of how much the RF power may be reduced before the performancefalls below an acceptable level. For example, if an acceptableperformance level is set to require that all RFID tags on a pallet mustbe read successfully at least 15 times for every 20 attempts, then theperformance margin may be expressed as the change in reader power levelat which that threshold is crossed. In this example, if all otherparameters are equal, the tags that have the least sensitivity determinethe performance margin. Accordingly, information about the sensitivityof individual tags provides an additional parameter that may be analyzedto identify the source of RFID performance errors.

For example, the performance monitoring system 100 may analyze tagsensitivity and other performance-related data and determine that areader has a performance margin that is trending down over time. Thesystem 100 may trigger an alarm or report that indicates thatmaintenance procedures should be taken to determine if the reader needsrepair or replacement. By taking such corrective action before thereader performance margin falls below zero, downtime and trackingfailures may be avoided, thereby saving time and money while maintaininghigh levels of RFID system performance.

In addition to providing sensitivity information that may be correlatedwith other performance-related information, the sensitivity informationmay also be combined with lot, manufacturer, version, and other taginformation, for example, that may be stored in the database 22. With orwithout performance-related information, this tag information may bemonitored as a quality control measure. For example, analysis may revealan undesirable sensitivity rate associated with a particular lot code,version, antenna design, or other tag-related parameter. Once a problemwith tag sensitivity has been identified, corrective action may betaken. For example, either the RFID system 10 or the system 100 may senda signal to the WMS 38 and/or to the IT Firm 40, which may be receivedby the purchasing group in the Firm A. The purchasing group may takeappropriate steps to change the procurement of unsatisfactory tags.Other related departments of Firm A, such as accounting, may beappraised that ability to track certain orders has been degraded as aresult of low tag sensitivity or failure corresponding to particularproducts moving through the supply chain. Downstream customers may benotified of the tag sensitivity issue, as well. Furthermore, operatorsmay be instructed to discontinue use of such tags, or to add redundanttags so that product tracking is not compromised.

Accordingly, determination of tag sensitivity and monitoring tagsensitivity in correlation with other parameters that may impact RFIDsystem performance may provide early detection and enable correctiveaction not available without the tag sensitivity information. As such,sensitivity information may be used directly to enhance RFID systemperformance, and to enhance the accuracy of the analysis ofperformance-related information.

In addition to the above-described examples, performance monitoring ofRFID systems may be implemented using systems, methods, or computerprogram products in embodiments other than the examples described above.

For example, by tracking the performance of specific productconfigurations within pallets, the system may be able to provide usefulinformation on preferred product arrangements for RFID systemperformance. The various product configurations that may be loaded on acase or a pallet may also have an impact on performance. Pallets may behomogeneous (single product) or mixed (more than one product). Thecombination of placement, orientation, materials, and the like, mayaffect the transmission of the reader RF signals to tags, and theresponse signal from the tag to the reader. In this regard, certainproduct configurations may perform better than others may. Accordingly,the system may collect information necessary to make such adetermination.

The RFID performance monitoring system 100 may be implemented as acomputer system that can be used with embodiments of the invention. Theprocessor 110 is capable of processing instructions for execution withinthe system 100. In one embodiment, the processor 110 is asingle-threaded processor. In another embodiment, the processor 110 is amulti-threaded processor. The processor 110 is capable of processinginstructions stored in the memory 120 or on a storage device.

The memory 120 stores information within the system 100. In variousembodiments, the memory 120 may be contained in a computer-readablemedium, a volatile memory, or a non-volatile memory. The system may alsoinclude a storage device capable of providing mass storage for thesystem 100. In various embodiments, the storage device may be acomputer-readable medium, a floppy disk device, a hard disk device, anoptical disk device, or a tape device.

The display 452 may be an input/output device that provides input/outputoperations for the system 100. In embodiments, an input/output devicemay include a keyboard and/or pointing device, or a display unit fordisplaying graphical user interfaces.

The invention can be implemented in digital electronic circuitry, or incomputer hardware, firmware, software, or in combinations of them.Apparatus of the invention can be implemented in a computer programproduct tangibly embodied in an information carrier, e.g., in amachine-readable storage device or in a propagated signal, for executionby a programmable processor; and method steps of the invention can beperformed by a programmable processor executing a program ofinstructions to perform functions of the invention by operating on inputdata and generating output. The invention can be implementedadvantageously in one or more computer programs that are executable on aprogrammable system including at least one programmable processorcoupled to receive data and instructions from, and to transmit data andinstructions to, a data storage system, at least one input device, andat least one output device. A computer program is a set of instructionsthat can be used, directly or indirectly, in a computer to perform acertain activity or bring about a certain result. A computer program canbe written in any form of programming language, including compiled orinterpreted languages, and it can be deployed in any form, including asa stand-alone program or as a module, component, subroutine, or otherunit suitable for use in a computing environment.

Suitable processors for the execution of a program of instructionsinclude, by way of example, both general and special purposemicroprocessors, and the sole processor or one of multiple processors ofany kind of computer. Generally, a processor will receive instructionsand data from a read-only memory or a random access memory or both. Theessential elements of a computer are a processor for executinginstructions and one or more memories for storing instructions and data.Generally, a computer will also include, or be operatively coupled tocommunicate with, one or more mass storage devices for storing datafiles; such devices include magnetic disks, such as internal hard disksand removable disks; magneto-optical disks; and optical disks. Storagedevices suitable for tangibly embodying computer program instructionsand data include all forms of non-volatile memory, including by way ofexample semiconductor memory devices, such as EPROM, EEPROM, and flashmemory devices; magnetic disks such as internal hard disks and removabledisks; magneto-optical disks; and CD-ROM and DVD-ROM disks. Theprocessor and the memory can be supplemented by, or incorporated in,ASICs (application-specific integrated circuits).

To provide for interaction with a user, the invention can be implementedon a computer having a display device such as a CRT (cathode ray tube)or LCD (liquid crystal display) monitor for displaying information tothe user and a keyboard and a pointing device such as a mouse or atrackball by which the user can provide input to the computer.

The invention can be implemented in a computer system that includes aback-end component, such as a data server, or that includes a middlewarecomponent, such as an application server or an Internet server, or thatincludes a front-end component, such as a client computer having agraphical user interface or an Internet browser, or any combination ofthem. The components of the system can be connected by any form ormedium of digital data communication such as a communication network.Examples of communication networks include, e.g., a LAN, a WAN, and thecomputers and networks forming the Internet.

The computer system can include clients and servers. A client and serverare generally remote from each other and typically interact through anetwork, such as the described one. The relationship of client andserver arises by virtue of computer programs running on the respectivecomputers and having a client-server relationship to each other.

In one embodiment, the system may, for example, automatically alert anoperator if a particular parameter is determined to have fallen out of apredetermined range or crosses a predetermined threshold limit. Inanother embodiment, the system may be configured to take correctiveaction, such as closing certain dock doors, signaling keep-out areas,generating orders relating to the purchase of tags, and turning on/offair conditioning equipment (e.g., HVAC, fans). The system may beconfigured to display parameters, trends, and correlation data forinterpretation by an operator.

In other embodiments, the monitoring may involve multiple input signalsthat provide information about the performance of an RFID system. In aparticular embodiment, this performance monitoring may be achieved, inpart, by collecting information from a number of reader stations in theRFID system. Additional sensors, such as environmental temperature andhumidity, may achieve further information collection. Once collected,such information may be analyzed by the performance monitoring system tostatistically correlate system performance with the collectedinformation, including time. One of the monitored parameters may includetime. By identifying relationships between RFID system performance andenvironmental parameters, solutions may be targeted to mitigate theireffects.

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention. Forexample, advantageous results may be achieved if the steps of thedisclosed techniques were performed in a different sequence, ifcomponents in the disclosed systems were combined in a different manner,or if the components were replaced or supplemented by other components.The functions and processes (including algorithms) may be performed inhardware, software, or a combination thereof. Accordingly, otherembodiments are within the scope of the following claims.

1. A method of collecting performance information for individual radiofrequency identification (RFID) tags, the method comprising: measuringthe performance of a tag; storing one or more values that represent themeasured performance in an information repository; and associating thestored values with an identifier that is uniquely associated with thetag.
 2. The method of claim 1, wherein measuring the performance of thetag comprises: stimulating the tag with radio frequency (RF) signals atleast once at each of a plurality of field strengths; in response tostimulation at each of the plurality of field strengths, attempting toread the tag; and determining a first field strength at which the tag issuccessfully read at an error rate that is less than a firstpredetermined error rate.
 3. The method of claim 2, further comprising:determining a second field strength at which the tag is not successfullyread at an error rate that is less than the first predetermined errorrate.
 4. The method of claim 3, wherein the first and second determinedfield strengths differ by less than a first predetermined amount, and afirst of the values that represent a measured performance is based onthe first and second determined field strengths.
 5. The method of claim4, wherein measuring the performance of the tag further comprises:determining a third field strength at which the tag is successfully readat an error rate that is less than a second predetermined error rate;and determining a fourth field strength at which the tag is notsuccessfully read at an error rate that is less than the secondpredetermined error rate, wherein the third and fourth determined fieldstrengths differ by less than a second predetermined amount, and asecond of the values that represent the measured performance is based onthe third and fourth determined field strengths.
 6. The method of claim2, wherein stimulating the tag with RF signals at least once at each ofa plurality of field strengths comprises reading the tag at least onceat each of a plurality of different spatial positions relative to areader.
 7. The method of claim 6, wherein reading the tag at least onceat each of a plurality of different spatial positions relative to thereader comprises placing the tag at a plurality of different locations.8. The method of claim 6, wherein reading the tag at least once at eachof a plurality of different spatial positions relative to the readercomprises placing the tag in a plurality of different spatialorientations.
 9. The method of claim 2, wherein stimulating the tag withRF signals at least once at each of a plurality of field strengthscomprises varying the power level transmitted by a reader.
 10. Themethod of claim 2, wherein reading a tag successfully at an error ratethat is less than a first predetermined error rate comprises reading thetag without errors at least a first predetermined number of times when asecond predetermined number of attempts are made to read the tag. 11.The method of claim 1, further comprising: encoding the tag with a codethat represents the identifier that is uniquely associated with the tag.12. An RFID reader that performs the method of claim
 1. 13. The methodof claim 1, wherein measuring the performance of the tag comprises:measuring a strength of an RF signal transmitted by the tag in responseto the tag receiving an RF signal associated with an attempt to read thetag; identifying one or more compensation values based on measurementsof one or more golden tags, each golden tag having a predeterminedstrength; and determining the one or more values that represent themeasured performance by applying the identified compensation values tothe measured strength.
 14. A computer program product (CPP) tangiblyembodied in an information carrier, the computer program productcontaining instructions that, when executed, cause a processor toperform operations to determine the sensitivity of a radio frequencyidentification (RFID) tag, the operations comprising: measure thesensitivity of a tag by stimulating the tag with radio frequency (RF)signals at least once at each of a plurality of field strengths andattempting to read the tag at each of the plurality of field strengths;store one or more values that represent the measured sensitivity in aninformation repository; and associate the stored values with anidentifier that is uniquely associated with the tag.
 15. A method ofcommissioning a radio frequency identification (RFID) tag for use in anRFID system that tracks the movement of article through a supply chain,the method comprising: measuring the performance of an individual RFIDtag; encoding the RFID tag with a unique identifier; associating theunique identifier with a first information representing the measuredperformance; storing the first information and the unique identifier inan information repository; and coupling the tag to an article to betracked as the article moves through the supply chain.
 16. The method ofclaim 15, wherein measuring the performance of an individual RFID tagcomprises: stimulating the tag with a radio frequency (RF) signal atleast once at each of a plurality of field strengths and attempting toread the tag at each of the plurality of field strengths; in response tostimulation at each of the plurality of field strengths, attempting toread the tag; determining a first field strength at which the tag issuccessfully read at an error rate that is less than a firstpredetermined error rate; and determining a second field strength atwhich the tag is not successfully read at an error rate that is lessthan the first predetermined error rate, wherein the first and seconddetermined field strengths differ by less than a first predeterminedamount, and a first of the values that represent a measured performanceis based on the first and second determined field strengths.
 17. Themethod of claim 15, wherein measuring the performance of an individualRFID tag comprises: measuring the performance of the tag after the taghas been coupled to the article to be tracked.
 18. The method of claim15, further comprising: estimating a power level sufficient for one ofthe RFID readers to read the tag based on the stored first information;and adjusting a reader power level according to the estimated powerlevel.
 19. The method of claim 18, wherein the estimated power level issufficient to read the least sensitive tag in a group of tags.
 20. Themethod of claim 18, wherein estimating a power level sufficient for oneof the RFID readers to read the tag based on the stored firstinformation comprises: determining a first power level that representsthe minimum reader power level required to successfully read the tag atan error rate that is less than the first predetermined error rate;determining the estimated power level by adding a predetermined marginto the determined first power level.